POLARIZATION SCIENCE

Major scientific areas that will benefit from excellent polarization capabilities of the ALMA include the following: Star formation. Theoretical and observational work have shown that magnetic fields can play a significant and perhaps essential role in the formation of interstellar clouds, in their evolution, and in the star formation process. Needed are observations of the morphology and strength of magnetic fields in molecular clouds. Techniques available include: (1) measurement of linearly polarized emission from dust grains aligned by magnetic fields; (2) measurement of linearly polarized spectral line emission (both in thermal lines due to the Goldreich-Kylafis effect and in maser lines such as SiO); and (3) measurement of circularly polarized spectral-line emission produced by the Zeeman effect. The first two techniques yield information about the morphology of magnetic fields in the plane of the sky, while the third gives the magnitude of the line of sight component of the field. Supernova remnants. Synchrotron emission from SNRs is linearly polarized, and the polarization is used to measure the direction and estimate the strength of magnetic fields. Normal galaxies. Synchrotron emission from the interstellar medium in normal galaxies may be used to map magnetic fields in external galaxies and study the morphology and estimate the strengths of extragalactic magnetic fields. Such studies may lead to an understanding of the amplification of magnetic fields in galactic dynamos. Radio galaxies. Radio lobes produce polarized synchrotron emission that may be used to map the morphology and estimate the strength of magnetic fields. Circular polarization observations will probably be primarily Zeeman line work carried out for that special purpose at a small number of frequencies. Certainly the 3-mm CN lines, and perhaps the CCS line at 33 GHz, the 1-mm CN lines, and several SO lines would be of interest. Other lines may of course also prove to be useful as the tremendous sensitivity of the ALMA is exploited. Except for the Zeeman effect, all of the above science drivers for polarization observations with the ALMA involve linear polarization. Requirements on the instrumental polarization are much more severe for continuum linear polarization mapping than for Zeeman observations. Moreover, for many if not most of the observations that will be made with the ALMA, the polarization of thermal dust continuum or synchrotron emission will be of scientific value EVEN WHEN THE POLARIZATION DATA ARE NOT THE PRIMARY PURPOSE OF THE OBSERVATIONS. Thus, optimization of instrumental characteristics of ALMA for routine linear polarization observations would be of the greatest scientific value.